1. Field of the Invention
The present invention relates to a process for producing a superconducting thick film, more particularly, it relates to a process for producing a superconducting thick film which exhibits a higher critical temperature of superconductivity and a reduced smaller discrepancy between the critical temperature and the onset temperature where phenomenon of superconductivity is started to be observed.
In this specification, "Tc" stands for the critical temperature of superconductivity and "Tcf" stands for the onset temperature where phenomenon of superconductivity is started to be observed.
2. Description of the Related Art
Under the superconducting condition, the perfect diamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a mean for delivering electric power without loss.
The superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2 K. at most. This means that liquidized helium (boiling point of 4.2 K.) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for other superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 to 50 K. which are extremely higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p.=20.4 K.) or liquidized neon (b.p.=27.3 K.) as a cryogen which bring them to exhibit the superconductivity.
It was also reported in the paper that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90 K. in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
However, the abovementioned superconducting materials of perovskite type oxides or quasi-perovskite type oxides which was just born have been studied and developed only in a form of sintered bodies or as in a form of bulk produced from powders. Therefore, they are inconvenient for handling, because the sintered bodies are fragile and hence are apt to be broken by even very weak mechanical load. In other words, they can not be utilized in practice in the form of a fine wire which is required in the field of power transmission.
Therefore, an object of the present invention is to solve the abovementioned problems and to provide a new superconducting material which exhibit higher "Tc" and "Tcf" and possessing improved mechanical strength.